1. Field of the Invention
The present invention relates to an image forming apparatus that moves a print sheet in response to a vertical synchronizing signal for forming a predetermined image, for example, a color video printer printing out an image in synchronization with a vertical synchronizing signal of a video signal.
2. Description of the Background Art
A conventional color video printer will be described hereinafter as a typical image forming apparatus. A color video printer stores desired image data applied by an input device such as a video deck into an image memory. The image data is transferred to a monitor output from the image memory in synchronization with a vertical synchronizing signal of a video signal. A printing operation is carried out by transferring image data from the image memory to a printing unit. The transfer timing of image data in a printing operation includes the method of using the output of a vertical synchronizing signal and the method of detecting the leading edge of a sheet with a sensor during transportation of a sheet. These two methods will be described in detail hereinafter.
The first method using an output of a vertical synchronizing signal will be described with reference to the block diagram of FIG. 11 showing the structure of a system including a conventional video printer.
Referring to FIG. 11, the system includes an input device 30, a video printer 20, and a monitor 31. Input device 30 provides image data to video printer 20. Video printer 20 provides the image data as a monitor output signal to monitor 31 in synchronization with a vertical synchronizing signal.
Video printer 20 includes an image memory 21, a printer unit 23, and a sheet convey unit 24.
Image data provided from input device 30 is input to image memory 21 to be stored therein. Image memory 21 provides image data to sheet convey unit 24, printer unit 23 and monitor 31 in synchronization with a vertical synchronizing signal. Sheet convey unit 24 responds to an input vertical synchronizing signal to convey a print sheet. Printer unit 23 responds to an input vertical synchronizing signal to print one line of data out of the input image data sequentially onto a print sheet. Sheet convey unit 24 is subject to an open loop control using a stepping motor that facilitates positioning since the start position for printing must be adjusted.
The second method carried out by detecting the leading edge of a sheet during transportation thereof will be described with reference to the block diagram of FIG. 12. Referring to FIG. 12, a system including a conventional video printer includes an input device 30, a video printer 20a, and a monitor 31. The operations of input device 30 and monitor 31 are similar to those of FIG. 11.
Video printer 20a includes an image memory 21a, a sheet sensor 22, and a printer unit 23. Image memory 21a stores image data provided from input device 30. Image memory 21a provides the stored image data to monitor 31. Sheet sensor 22 provides a sheet edge detection signal to image memory 21a when one edge of a print sheet to be transported is detected. Image memory 21a responds to a sheet edge detection signal to move the image data to printer unit 23. Printer unit 23 forms a predetermined image on a print sheet according to the transferred image data. Here, control of the start position of printing is not required, and transportation of a print sheet is carried out using a stepping motor, a DC motor, and the like.
The above-described method using an output of a vertical synchronizing signal for the transfer timing of image data is disadvantageous over the case where a DC motor is used since the employed stepping motor is bulky with inferior efficiency of the motor itself. The usage of a stepping motor was a bottleneck to reduction in the size and power consumption of the apparatus.
In the above-described second method, a transfer path to printer unit 23 must be provided in addition to the monitor output in the transfer system of image memory 20. This increases the circuit complexity, as well as the image data transfer control complexity. The transfer rate and processing rate of image data depend upon the ability of image memory 21a, so that it was difficult to realize a highly functional video printer.
In order to circumvent these difficulties, research efforts have been made to control the image data transfer timing using a vertical synchronizing signal. However, a sheet edge detection signal provided from a sensor detecting one edge of a print sheet and a vertical synchronizing signal are independent of each other. Furthermore, a color video printer employs the method of reproducing color image information generally with 3 colors superimposed. These two facts yield a problem set forth in the following. FIG. 13 is a timing chart for describing the problem of a conventional video printer. Referring to FIG. 13, the time from a rise of a sheet edge detection signal to an input of a vertical synchronizing signal is .DELTA.t1, .DELTA.t2, and .DELTA.t3 for the first color, second color, and the third color, respectively. The term from the rise of a sheet edge detection signal to an input of a vertical synchronizing signal differs for each color. This difference leads to deviation in the printing position, resulting in offset in color registration. Such a deviation in the printing position was as great as a shift corresponding to 1 period of a vertical synchronizing signal at the maximum. It was therefore difficult to reproduce a color image in good registration. Furthermore, the structure of the control circuit becomes more complicated if the above process is to be carried out. There was the problem that the sheet transportation control operation is difficult and unstable.